1. Field of the Invention
This invention relates to twist drills and methods for their manufacture. More particularly, the invention relates to an improved type of drill bit, and to a novel method for making the improved drill.
2. Description of Background Art
Twist drills in a wide range of sizes have long been used in the fabrication of a large variety of manufactured products. One industry which now employs prodigious quantities of twist drill bits, or drills, is the electronics industry. The drills are used chiefly to make holes in printed circuit boards through which the leads of electronic components are inserted. Typical printed circuit boards used in relatively simple consumer electronic equipment may require the drilling of 100 to 5,000 holes, in sizes ranging from 0.004 inch to 0.250 inch in diameter. More complex consumer electronic equipment, and industrial and military electronics apparatus, may employ many printed circuit boards, some of which may require drilling as many as 50,000 holes of various sizes through the board.
The drilling of printed circuit boards imposes some rather special demands on drill bits used for that purpose.
First, the small size of the holes which must be drilled in printed circuit boards requires that drills as small as 0.004 inch in diameter be used to make the holes. Thus, even when made of a high strength material such as steel or carbide, drills of such small diameter tend to break relatively easily. The costs of purchasing replacement bits and the additional costs resulting from production down-time for drill replacement, could be reduced if breakage of small drill bits could be made more infrequent.
A second demand placed on drills used in the manufacture of printed circuit boards arises from the nature of the materials used to fabricate the boards. A significant percentage of printed circuit boards is fabricated from a composite material. For example, some printed circuit boards are made of thermosetting resin such as expoxy, in which glass fibers are imbedded. The glass fibers are hard and abrasive. Thus, drills intended for use with such printed circuit boards must be made from a very hard material, such as high-carbon steel, or carbide. Otherwise, the drills would dull quickly, requiring excessively frequent replacement or re-sharpening.
Manufacturing drill bits from a hard or refractory material such as high-carbon steel or tungsten carbide solves the problem of providing a drill which can form an acceptable number of holes in fiberglass-expoxy printed circuit boards before requiring re-sharpening. However, varying the composition of the drill to make it harder has the undesirable side effect of making the drill more brittle, at least with any presently known materials. Thus, making a drill harder to avoid premature dulling makes the drill more susceptible to breakage.
A third demand placed on drills used for printed circuit board fabrication arises from another characteristic of the materials from which printed circuit boards are made. The frictional heat generated by the drilling process is often sufficient to melt some of the resin matrix of a printed circuit board. The melted resin has a tendency to flow radially inwards into a freshly drilled hole, a problem referred to as "resin smear." Then, if a drill bit having a uniform body diameter, or one which tapers to a somewhat smaller body diameter near the front edge or cutting lip of the drill, is withdrawn from the hole, the body of the drill rubs the wall surrounding the hole. Rubbing the hole wall during drill withdrawal can cause the hole diameter to be non-uniform, and can also result in broken drill bits and additional wall heating and resin smear.
To prevent side wall rubbing, most drill bits used in the printed circuit board industry are "back tapered." In a back tapered drill, the body of the drill tapers rearward to a small diameter "root", i.e., junction with larger diameter shank which is held in the chuck of the drilling machine. But the smaller diameter of the root makes the drill more susceptible to breakage at that point.
Some printed circuit board drills are undercut to provide a space or relief between the body of the drill and the wall of a newly drilled hole. In an undercut drill, a substantial portion of the length of the body of the drill rearward of the tip is of smaller diameter than the tip of the drill. Again, the reduced diameter root of such undercut drills makes them more susceptible to breakage than drills which are front tapered, or are of uniform diameter.
Jeremias, in U.S. Pat. No. 4,480,952, Nov. 6, 1984, Non-burring Drill For Composite Materials, discloses a drill having a pointed blade-like tip portion extending forward from a cylindrical base having the same diameter as the shank of the drill. An annular groove rearward of the base having transverse forward and rearward wall surfaces forms therewith a right angled, circular cutting edge. The blade-like tip portion is provided for initiating a hole in a composite workpiece. The circular, rear facing cutting edge is provided for removing fibers projecting from the walls of the hole upon retracting the drill through the hole. Neither the Jeremias patent, nor any other prior art which the present inventor is aware of, discloses a drill bit for printed circuit boards which provides a satisfactory solution to the problems described in the previous paragraphs.